1. Field of the Invention
This invention relates to a slit exposure type optical scanning device having a scanning system on the image side with an original and a fixed lens.
2. Description of the Prior Art
Among slit exposure scanning type copying machines and microfilm printers, some have a scanning system in the object side area and some have a scanning system in the image side area. The object side area is an area in the optical path from the original to the lens, and the image side area is an area from the lens to the photosensitive medium. Generally, if the scanning system is in the object side area, it is necessary to vary the speed of movement of the scanning system in accordance with the copying magnification, but if the scanning system is in the image side area, the speed of movement of the scanning system can be made constant independently of the copying magnification.
In a conventional example disclosed in British Pat. No. 1,223,449 which has a scanning system in the image side area, the number of mirrors forming the scanning system is one and the vicinity in which the principal ray is coincident with the optical axis cannot be used and therefore, only one half of the angle of view of the lens system can be used and accordingly, the use of such a scanning system has encountered the problem that a lens having a wide angle of view is required.
In the scanning system disclosed in Japanese Laid-open Patent Application No. 102041/1978, a lens whose maximum angle of view for the optical axis is about one half of that of the lens shown in the aforementioned British Pat. No. 1,223,449 suffices and scanning is carried out only by rectilinear movement and thus, the mechanism is simple. FIG. 1 of the accompanying drawings shows such scanning system. In FIG. 1, the slit area of a fixed original 1 is projected onto a drum 5 rotated in the direction indicated by the arrow, by a lens 2 via mirrors 3 and 4. In the optical path, short of the drum 5, there is provided a light beam controlling member 6 for controlling the slit area. In such a system, if the mirrors 3 and 4 are moved together in the direction of the moving tangential line of the drum 5 at 1/2 of the speed of the drum 5, the slit area of the original 1 is successively scanned and the whole original is copied with the original 1 and the lens 2 remaining fixed. However, the number of mirrors of this scanning system is two and the system is of such a construction that the emergent principal ray turns back the incident principal ray, and the drum 5 and the lens 2 are spatially close together and this leads to a problem that there is no room for the devices provided around the drum. Further, if an attempt is made to widen the spacing between the lens and the drum to provide such room the problem will occur that when the mirror 4 closer to the drum moves away from the lens in the optical path, the angle of view of the effective light beam becomes wider and thus the system becomes bulky. Also, if the scanning system is moved up and down or, vibrated in the direction of the incident optical axis, the problem that the length of the optical path is varied will arise .